Sound absorbers have been widely used in a number of different disciplines for absorbing sound. The most common sound absorbers are fiber-based and use fibrous materials such as fiberglass, open-cell polymeric foams, fibrous spray-on materials often derived from polyurethanes, and acoustic tile (an agglomerate of fibrous and/or particulate materials). Such fibrous-based sound absorbers rely on frictional dissipation of sound energy in interstitial spaces and can advantageously provide relatively broad-band sound absorption. Despite their advantages in broad-band absorption, fiber-based sound absorbers have significant inherent disadvantages. Such sound absorbers can readily release particulate matter and deleteriously degrade the air quality of the surrounding environment. Some fiber-based sound absorbers are also sensitive to heat or fire and/or require expensive treatment to provide heat/fire resistance. Consequently, fiber-based sound absorbers are of limited use in many environments.
Perforated sheets have also been used in sound absorbers. Typically, these sheets include relatively thick perforated material, such as metal, having relatively large hole diameters (e.g., greater than 1 mm hole diameters). The perforated sheets are commonly used in two manners. They are often used alone with a reflective surface to provide narrow band sound absorption for relatively tonal sounds. They are also used as facings for fibrous materials to provide sound absorption over a wider spectrum. In the later case, the perforated sheets typically serve as protection, with the fibrous materials providing the sound absorption. Microperforated, sheet-based sound absorbers have also been suggested for sound absorption. Conventional micro perforated sheet-based sound absorbers use either relatively thick (e.g., greater than 2 mm) and stiff perforated sheets of metal or glass or thinner perforated sheets which are provided externally supported or stiffened with reinforcing strips to eliminate vibration of the sheet when subject to incident sound waves.
Fuchs, U.S. Pat. No. 5,700,527, for example, teaches a sound absorber using relatively thick and stiff perforated sheets of 2-20 millimeter glass or synthetic glass. Fuchs suggests using thinner sheets (e.g., 0.2 mm thick) of relatively stiff synthetic glass provided the sheets are reinforced with thickening or glued on strips in such a manner that incident sound cannot exite the sheets to vibrate. In this case the thin, reinforced sheet is positioned 24 inches from an underlying reflective surface. Mnich, U.S. Pat. No. 5,653,386, teaches a method of repairing sound attenuation structures for aircraft engines. The sound attenuation structures commonly include an aluminum honeycomb core having an imperforate backing sheet on one side, a perforated sheet of aluminum (with aperture diameters of about 0.039 to 0.09 inches) adhered to the other side, and a porous wire cloth adhesively bonded to the perforated aluminum sheet. According to Mnich, the sound attenuation structure may be repaired by removing a damaged portion of the wire cloth and adhesively bonding a microperforated plastic sheet to the underlying perforated aluminum sheet. In this manner, the microperforated plastic sheet is externally supported by the perforated aluminum sheet to form a composite, laminated structure which provides similar sound absorption as the original wire cloth/perforated sheet laminated structure.
While these perforated sheet-based sound absorbers may overcome some of the inherent disadvantages of fiber-based sound absorbers, they are expensive and/or of limited use in many applications. For instance, the use of very thick and/or very stiff materials or use of thickening strips or external support for the perforated sheets limits the use of sound absorbers using such sheets. The necessary thickness/stiffness or strips/external support also makes the perforated sheets expensive to manufacture. Finally, the perforated sheets must be provided with expensive narrow diameter perforations or else used in limited situations involving tonal sound. For example, to achieve broad-band sound absorption, conventional perforated sheets must be provided with perforations having high aspect ratios (hole depth to hole diameter ratios). However, the punching, stamping or laser drilling techniques used to form such small hole diameters are very expensive. Accordingly, the sound absorption industry still seeks sound absorbers which are inexpensive and capable of wide use. The present invention solves these as well as other needs.